Analysis of the causes of capacitors between power supply and ground

PCB technology: the reason for connecting a capacitor between the power supply and the ground

1. The reason for connecting a capacitor between the power supply and the ground is that there are two functions, energy storage and bypass. Energy storage: The power consumption of the circuit is sometimes large and sometimes small. When the power consumption suddenly increases, if there is no capacitor, the power supply voltage will be pulled down, generating noise and ringing. In severe cases, it will cause the CPU to restart. At this time, a large-capacity capacitor can temporarily release the stored energy and stabilize the power supply voltage, just like the relationship between a river and a reservoir. Bypass: The circuit current often pulsates. For example, the synchronization frequency of a digital circuit will cause the power supply voltage to pulsate. This is an AC noise. A small-capacity non-polar capacitor can bypass this noise to the ground (capacitors can pass AC and block DC. The passband of small-capacity capacitors is much higher than that of large capacitors). This is also to improve stability.

2. Power supply filtering

Capacitance of capacitor = dielectric constant * area / distance = ε * S / d, usually ε, d is not easy to change, only S can be changed to change the capacitance. When the capacitor is very large, S must be large, in order to reduce the volume, it has to be wound, but the winding will inevitably increase the inductance (although symmetrical double winding). As you know, the capacitor is actually a combination of R, L, and C, so the large capacitor has a large relative inductance L. For example: when using a 2200uF capacitor wave, it is very good for low frequency 50Hz, but for high frequency (K, MHz), it is useless because L is too large. Therefore, experts pay attention to the filtering of the power supply, and will use large, medium and small capacitors to filter low, medium and high frequencies respectively. The following combinations are commonly used: 2200/47/0.1uF220/4.7/0.1uF1nf, 0.1u and 4.7uf.

1. Configuration of high frequency filter capacitors

A. For small-scale integrated circuits with less than 10 outputs , when the operating frequency is ≤50MHz, at least one 0.1μf filter capacitor should be connected. When the operating frequency is ≥50MHz, each power pin should be equipped with a 0.1μf filter capacitor.

B. For medium and large scale integrated circuits, each power pin is equipped with a 0.1μf filter capacitor. For circuits with large power pin redundancy, the number of matching capacitors can also be calculated according to the number of output pins, with a 0.1μf filter capacitor for every 5 outputs.

C. For areas without active devices, at least one 0.1μf should be connected for every 6cm2.

D. For ultra-high frequency circuits, each power pin is equipped with a 1000pf filter capacitor. For circuits with large power pin redundancy, the number of matching capacitors can also be calculated according to the number of output pins, with a 1000pf filter capacitor for every 5 outputs.

E. For dedicated circuits, refer to the filter capacitor configuration recommended in the application manual.

F. For circuits or areas with multiple power supplies, filter capacitors should be connected to each power supply according to 1, 2, and 3 respectively.

G. High-frequency filter capacitors should be placed as close to the power pins of the IC circuit as possible.

The connection from the filter capacitor pad to the connection pad should be connected with a 0.3mm thick wire, and the interconnection length should be ≤1.27mm

2. Configuration of low-frequency filter capacitors

A. For every 5 high-frequency filter capacitors, at least one 10μf low-frequency filter capacitor should be connected;

B. For every 5 10μf capacitors, at least two 47μf low-frequency filter capacitors should be connected;

TC. At least one 220μf or 470μf low-frequency filter capacitor should be connected within every 100cm2;

D. At least two 220μf or 470μf capacitors should be configured around each module power outlet. If space permits, the number of capacitors should be appropriately increased;

E. Low-frequency filter capacitors should be placed evenly around the circuit being filtered.

The schematic diagram is drawn together for the sake of appearance. On the PCB, it should be drawn at the chip power pin. Try to be close to the chip. There are a lot of them on the schematic diagram, and they should be assigned to the corresponding IC and power inlet on the PCB.